Non-absorbable, or just partially absorbable, bone cements have been known for a long time and have been described in a large number of patents and scientific publications (e.g. G. Lewis, J. Biomed. Mater. Res. (Appl. Biomater.) 38 (1997) 155). They usually consist of a liquid component and a solid component. The liquid component consists of a liquid monofunctional methacrylic acid ester, with preference being given to the use of methyl methacrylate. A polymerisation activator, usually N,N-dimethyl-p-toluidine, is dissolved in that monomer. The solid component consists of a polymer which is swellable or soluble in the monomer, with copolymers of methyl methacrylate and of methylacrylate being most frequently used. Contained in the solid component is a polymerisation initiator, such as dibenzoyl peroxide. Curing of those bone cements is accomplished by means of the fact that the polymerisation activator dissolved in the monomer comes into contact with the polymerisation initiator contained in the solid component. As a result of. the action of the polymerisation activator, the polymerisation initiator decomposes immediately, giving rise to free radicals which immediately trigger polymerisation of the monofunctional monomer. As a result of polymerisation of the monofunctional monomers there are formed non-crosslinked polymers which are soluble or swellable in the as yet unreacted monomer. As a result, for a period of several minutes, the cement remains plastically deformable and can be worked. The polymerisation speed is dependent only on the initiator and monomer concentrations and therefore is relatively slow.
In the case of multi-functional monomers, those bone cements are no longer feasible because multi-functional monomers, by virtue of their polymerisation kinetics, polymerise extremely quickly and result in solid polymer networks which, even in the case of low degrees of crosslinking, can no longer be plastically deformed and therefore worked.
In U.S. Pat. No. 5,814,682 there is described a composition formed by a paste A, which consists of a mixture of a polymerisable monomer, an initiator and calcium phosphate, and a paste B, which consists of a mixture of a polymerisable monomer, an activator and calcium phosphate. After pastes A and B have been mixed together, the composition polymerises.
A composite system similar to U.S. Pat. No. 5,814,682 is described in WO 87100058. A bone cement based on diacrylate or dimethacrylate comprises absorbable particles of bioceramic material or bioglass. The inorganic filler particles should have a pore volume of at least 0.2 ml/g because this is said to have a beneficial effect on the physical properties of the cement.
A dental material capable of free radical polymerisation has been described in EP 0951896 A2. That dental material is characterised in that the filler is formed by a homogeneous mixture of a first part of the filler, which is coated with the polymerisation initiator, a second part of the filler, which is coated with the polymerisation activator, and a third part of the filler, which does not comprise any component of the initiator system.
A biologically degradable composite material is disclosed in DE 19939403 A1. The composite material is formed by curing of the mixture of a liquid component A, which comprises at least one polymerisable, bioabsorbable monomer and, optionally, a bioabsorbable thickener, a solid component B, which consists of a bioabsorbable inorganic filler coated with a polymerisation initiator, and a solid component C, which is formed by a bioabsorbable inorganic filler coated with a polymerisation activator. In that composite system, the polymerisation activator and the polymerisation initiator are applied as coatings to the surface of fillers. The inorganic filler may be calcium carbonate, magnesium carbonate, calcium phosphate or hydroxyapatite; there is no requirement for interconnecting porosity.
DE 198 18 210 A1 describes a dental material capable of free radical polymerisation, having at least one polymerisable binder and at least one filler and comprising a redox initiator system for the free radical polymerisation, the system comprising an initiator and an activator. In the known material, the filler is a homogeneous mixture of a first part of the filler, which is mixed with the initiator, a second part of the filler, which is mixed with the activator, and a third part of the filler, which does not comprise any component of the initiator system. Again, interconnecting porosity is not required for the filler, with mention being made of, for example, quartz powder, glass-ceramic powder, glass powder, aluminium powder and silicon dioxide powder as preferred fillers.
U.S. Pat. No. 5,814,681 relates to a restorative composition for hard tissue, comprising inorganic calcium phosphate powder; again, there is no requirement for interconnecting porosity. ω,-ω′-unsaturated compounds are provided as polymerisable monomers, the following warning being given for the polymerisation thereof:
“Excess time for mixing leads to the initiation of curing before application of the mixture to the affected part, thereby making the mixture unavailable.”
DE 44 35 860 A1 relates to production of a porous bone replacement material using an inorganic starting material, the porosity of which is not specified. For production of the known bone replacement material, a mixture of (a) a polymerisation product comprising polymerisation catalyst, (b) a liquid monomer comprising polymerisation accelerator and (c) inorganic material in the form of coarsely particulate granules is used as starting material.
DE 100 18 394 A1 relates to production of porous calcium phosphate pieces obtained by sintering.